Are literacy and numeracy less heritable than general cognitive ability? Monozygotic and dizygotic twins were assessed for literacy, numeracy, and general cognitive ability at ages 7, 9, and 12. Literacy and numeracy were found to be more heritable than general cognitive ability at ages 7 and 9 but not at age 12. The authors posit that widespread early education in numeracy and literacy reduces the impact of the environment on these skills so that variation between children depends more heavily on genetic differences. As children age, they begin choosing their own environments, which are often correlated with their genetic abilities, leading to the decrease in environmental influences on general cognitive ability observed in older children.

Past research has found that genetic effects for IQ are stronger in children with high socioeconomic status (SES) than in children with low SES. Is this still true in adulthood? When adult monozygotic and dizygotic twins were assessed for childhood SES and adult intelligence, the authors found that childhood SES amplified the effects of genes involved in adult intelligence. This finding supports a biological model of intelligence in which supportive environments lead to maximal genetic effects.

Studies have shown that genetic influences on intelligence increase as we age. The transactional model explains this increase by proposing that a person’s unique genetic makeup leads him or her to select (and be selected into) various types of environments that in turn reinforce gene-based behaviors over time. Tucker-Drob, Briley, and Harden discuss this model and the conditions under which this gene-environment relationship is strengthened or weakened.